Subj : DLL access to a singleton in Borland BCB6
To   : borland.public.cpp.borlandcpp
From : rtw1992
Date : Tue Apr 27 2004 08:56 pm

I have declared a singleton class in my main exe.  Now I want to use a
DLL which must also have access to the same singleton object.  Problem
is that I can't get it to compile, I don't see how to declare the
singleton object in the DLL unit.  Here's the cut down simplified code
for the 2 projects that shows my problem.  There are 4 files,
unit1.cpp and unit1.h make up the main executable, and DLLUnit1.cpp
and DLLUnit1.h make up the DLL.  With the way the code is right now,
when I build the DLL project I get

[Linker Error] Unresolved external 'MySingleton::getInstance()'
referenced from C:\DLLUNIT1.OBJ

Any suggestions?

//---------------------------------------------------
//Unit1.h
class MySingleton
{
public:
    static MySingleton* getInstance();
    void inc(){++count;}
    long getCount(){return count;}
private:
    MySingleton(){count=0;}
    static MySingleton* mp_Instance;
    long count;
};

//---------------------------------------------------
//Unit1.cpp
#include <vcl.h>
#include <iostream.h>

#include "Unit1.h"
#define _DLL_PROJECT_
#include "DLLUnit1.h"

//This is where the object is declared in the main exe
MySingleton* MySingleton::mp_Instance = NULL;

MySingleton* MySingleton::getInstance()
{
    if (NULL==mp_Instance)
    {
        mp_Instance= new MySingleton;
    }
    return mp_Instance;
}


int main(int argc, char* argv[])
{
    MyTestDLLFunction();
    cout << MySingleton::getInstance()->getCount() << endl;
    MyTestDLLFunction();
    cout << MySingleton::getInstance()->getCount() << endl;

    MySingleton::getInstance()->inc();
    cout << MySingleton::getInstance()->getCount() << endl;
    return 0;
}

//---------------------------------------------------
//DLLUnit1.h
#ifdef cplusplus
extern "C"
{
#endif

#ifdef _DLL_PROJECT_
    __declspec(dllexport) void MyTestDLLFunction();
#else
    __declspec(dllimport) void MyTestDLLFunction();
#endif

#ifdef cplusplus
}
#endif

//---------------------------------------------------
//DLLUnit1.cpp
#include "vcl.h"
#include "unit1.h"

#define _DLL_PROJECT_
#include "DLLunit1.h"

// How do I declare the following to say that it will be declared
// in the emain executable
MySingleton* MySingleton::mp_Instance;


void MyTestDLLFunction()
{
    MySingleton::getInstance()->inc();
}

.